This invention relates to ultracentrifuges and particularly to speed dampers for ultracentrifuges. Still more particularly, this invention relates to a solenoid-actuated ultracentrifuge damper that is diesngagable from the drive shaft when the rotational speed attains a predetermined critical value.
Ultracentrifuges are used to separate liquid materials of different densities and solids from liquids by rotating a mixture of materials in a tube at angular velocities of 100,000 revolutions per minute or more. The material having the greatest density, and, hence the greatest inertia will aggregate at the end of the tube furthest from the axis of rotation. If a plurality of materials of differing density are in the tube, they will become arranged in descending order of density toward the axis of rotation.
An important consideration in ultracentrifuge design is the necessity of minimizing stresses upon bearings ued in conjunction with high speed components such as the drive shaft that connects the rotor to the driving mechanism. It is common practice in the design and construction of an ultracentrifuge to make the drive shaft to have a relatively small diameter to provide a degree of flexibility in the drive shaft. Two primary reasons exist for requiring flexibility in the drive shaft.
First, when a user is operating an ultracentifuge rotor, it is very important to place test samples so as to have a balanced, symmetrical mass distribution about the drive shaft. However, perfect balance is usually impossible; and even small variations have deleterious effects on the operational characteristics of the ultracentrifuge system at angular velocities typically achieved in such systems because the centripetal force on any given mass is proportional to the square of the angular velocity. Even a very small imbalance could cause vibrations that are capable of applying damaging stresses to the high speed bearings that are required to support the shaft. A slight flexing of the drive shaft accommodates the imbalance and prevents application of undesirable stresses to the bearings.
A second reason for providing flexibility in the drive shaft relates to slight geometric limitations inherent in the machining processes used to form the rotor shaft and associated drive mechanism. It is impossible to construct an ideal drive shaft of uniform density and diameter, because there are always tolerances that must be allowed in forming the drive shaft. Furthermore, it is also impossible to perfectly align the drive shaft with the drive mechanism. Although ultracentrifuge components are machined to be very nearly perfect, the nature of the ultracentrifuging process is such that the slightest imbalance or misalignment will become apparent when the system is in use at high rotational speeds. The usual effect of an imbalance or misalignment is unacceptable wear on the drive shaft bearings, which as explained above is relieved by a flexible drive shaft.
However, the use of a thin, flexible drive shaft causes problems in the acceleration of the device to the high speeds required. It is well-known that a thin, elongate shaft rotating about its longitudinal axis has certain natural frequencies of vibration that become apparent at certain critical speeds. The lowest critical speed is a parameter of the centrifuge system and depends primarily upon the shaft stiffness and the rotor mass.
If only one end of the shaft is fixed, that end is always a node, and the free end is always an antinode at the resonant frequencies. In a typical ultracentrifuge, the first resonance occurs at an angular velocity of about 500 RPM. In general, the amplitudes of the second and higher order resonances are out of the operating speed range and have no effect upon the efficacy of ultracentrifuging processes or upon the high speed components of ultracentrifuge systems.
Ultracentrifuge operations require acceleration of the drive shaft to speeds greater than the speed at which the first resonance occurs. If the shaft is not sufficiently stiff, stabilized, or damped, the combination of vibrations caused by unbalanced conditions from the test samples and the structure of the rotor and the resonance may cause deflections of the shaft sufficient to cause damage to the centrifuge and remix the sample.
A possible solution to the difficulties caused by imbalances and resonances in the system is to fix a damper bearing on the thin drive shaft. Fixed dampers must be designed for both low speed and high speed operation and are, therefore, generally limited because of the additional complexity of the dynamics of such designs. Other attempts to solve the problems associated with low speed resonances include journalling the shaft in a plurality of bearings with the amount of bearing surface engaging the rotating shaft being adjustable.
U.S. Pat. No. 2,961,277, issued Nov. 22, 1960 to Sternlicht discloses a bearing system in which a shaft has a frustoconical journal portion intermediate the ends of the shaft, which are supported on fixed bearings. A bearing is mounted on an adjustable support to be movable into or out of engagement with the frustoconical journal. The movable bearing is engaged with the journal before the shaft reaches the critical angular velocity and is disengaged from the shaft after the angular velocity is greater than the critical value.
U.S. Pat. No. 4,205,779, issued June 3, 1980 to Jacobson and assigned to Beckman Instruments, Inc. assignee of the present invention, discloses an ultrcentrifuge drive system that includes a fixed damper bearing. jacobson discloses a cylindrical collar around the shaft. A solenoid actuated bushing having a tapered centering chamber is adapted to move into contact with the collar to laterally support the rotor.
U.S. Pat. No. 3,958,753, issued May 25, 1976 to Durland et al. discloses a centrifuge in which the rotor is driven by an air jet and supported on an air cushion. A solenoid moves a brake member into engagement with a friction bearing mounted on the bottom of the rotor to decelerate the rotor and provide stability to the rotor as it reduces its speed from a high rotational speed to come to rest.
U.S. Pat. No. 3,322,338 to Stallman et al. discloses a centrifuge having a movable bearing assembly carried by a frame that supports a rotatable member coaxially with the axis of rotation of the rotor. The rotatable member is movable between advanced and retracted positions to engage and release the rotor and is formed to engage the rotor to hold it in a defined axis of rotation. Stallman et al. further disclose means for permitting the rotatable member tomove laterally within predetermined limits, thereby damping lateral rotor movement at critical transition speeds.
U.S. Pat. No. 2,951,731 to Rushing discloses a centrifuge having damping means including two sets of concentric, spaced apart cylindrical sleeves. The sleeves are arranged to follow shaft vibrations and overlap with other sleeves that are fixed with respect to the shaft. A viscous liquid is retained between the overlapping sleeves to damp out shaft vibrations.
U.S. Pat. No. 3,902,659 to Brinkman et al. discloses a rotor stabilizing device having an upper bearing formed of a first axially polarized magnetic ring and a second ring including a ferrite material. One of the rings is secured to the rotor, and the other ring is held stationary relative to the rotor. The rings are positioned such that oscillations of the rotor cause eddy currents in aninduction ring, which absorbs the vibrations.
U.S. Pat. No. 3,786,694 to Willeitner discloses a damping device for a centrifuge rotor that is elastically supported by hydraulic oil. The damping device comprises a plurality of coaxial ring magnets and a disc that damp rotor vibrations in the oil.
U.S. Pat. No. 3,430,852 to Lenkey et al. discloses a centrifuge rotor stabilizing device that frictionally contacts the rotor to provide stability at critical speeds.
International application No. PCT/US83/00402 of Beckman Instruments, assignee of the present application, discloses a centrifuge stabilizing bearing that is actuated by a solenoid in response to a specified rotational speed for engagement with a bearing mounted to the rotor.